These studies are designed to isolate and characterize the different classes of monocyte precursors present in bone marrow. We will also investigate the factors that control the supply, proliferation, differentiation and activation of monocytes under normal steady state and pathological conditions. We will fractionate bone marrow cells based on their size and density. Various classes of monocyte precursors will then be defined and classified according to their behavior in tissue culture. To "activate" monocytes, we will expose them to different conditions and stimuli: adherence, phagocytosis, lymphokine and colony-stimulating factor. To identify "activated monocytes" we will use as our criteria the ability for anchorage independent growth and the enhanced production of plasminogen activator. We will study the effect of inflammation and infection on monocyte precursors in bone marrow.